Add a pull request template (nvim-lua#825)

Author: dam9000

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+***************************************************************************
+**NOTE**
+Please verify that the `base repository` above has the intended destination!
+Github by default opens Pull Requests against the parent of a forked repository.
+If this is your personal fork and you didn't intend to open a PR for contribution
+to the original project then adjust the `base repository` accordingly.
+**************************************************************************
+